Контрол на грешките

Въведение

Errorcontrolisatechnologythatusescodingmethodstocontrolerrorsgeneratedduringtransmissionindigitalcommunicationtoimprovetheaccuracyandeffectivenessoftransmission.Errorcontrolincludeserrordetection,forwarderrorcorrection(FEC)andautomaticretransmissionrequest(ARQ).

Accordingtothedifferentnatureoferrors,errorcontrolisdividedintoerrorcontrolforrandomerrorsanderrorcontrolforbursterrors.Randomerrorsmeanthatchannelerrorsaremoreevenlydistributedindifferenttimeintervals;whilebursterrorsmeanthatchannelerrorsareconcentratedinashortperiodoftime.Sometimesseveralerrorcontrolmethodsaremixedandused,andacertainerrorcontrolcapabilityisrequiredforrandomerrorsandbursterrors.

Amethodtoensurethatthereceiveddataiscompleteandaccurate.Becausetheactualtelephonelineisalwaysimperfect.Datamaybecomedisorderedorlostduringtransmission.Inordertocatchtheseerrors,thesendingmodemperformsamathematicaloperationonthedatatobesentandsendstheresultoftheoperationtogetherwiththedata.Thereceivingmodemperformsthesameoperationonthedataitreceivesandcomparesthetworesults..Ifthedataisdamagedduringtransmission,thetworesultsareinconsistent,andthemodemreceivingthedatarequeststhesendertoresendthedata.

Класификация на грешките

Errorsinthecommunicationprocesscanberoughlydividedintotwocategories:oneisrandomerrorscausedbythermalnoise;theotherisburstscausedbyconflictnoisemistake.Suddenerrorsaffectthelocalarea,whilerandomerrorsaffecttheoverallsituation.

Причини

Thecausesoferrorsaremainlyduetotherandomnoise(thermalnoise)generatedbytheelectricalcharacteristicsofthelineitself,theattenuationordistortionofthesignalamplitude,frequencyandphase,andtheelectricalsignalThereflectionechoeffectonthetransmissionmedium,thecrosstalkofadjacentlines,externalelectromagneticinterferenceandequipmentfailurearecausedbyfactors.

Метод за контрол на грешките

Общ метод

Производителността в реално време е добра и е възприета опростената комуникация.

2,automaticrepeatrequest(ARQ).Emphasizeserrordetectioncapability,doesnotrequireerrorcorrectioncapability,andadoptsbidirectionalchannels.

3.Hybriderrorcorrection.Theabovetwomethodsareintegrated,butthetransmissionequipmentisrelativelycomplicated.

Error control

Класификационен метод

Errordetectionisthebasisoferrorcontrol.Codesthatcancorrecterrorsshouldfirsthaveerrordetectioncapabilities,andonlywhenitcandeterminewhetherthereceivedsignalisinerrorcanitbepossibletoasktheotherpartytoresendtheerrormessage.Acodewitherrordetectioncapabilitydoesnotnecessarilyhaveerrorcorrectioncapability.Becauseerrordetectioncannotimprovechannelutilization,itismainlyusedonchannelswithbettertransmissionconditionsasameansoferrorstatisticsandqualitycontrol.

AutomaticrequestforretransmissionofARQandforwarderrorcorrectionFECaretwomethodsforerrorcontrol.

InARQmode,whenthereceivingenddetectsanerror,ittriestonotifythesendingendtoretransmituntilthecorrectcodewordisreceived.TheARQmethoduseserrordetectioncodes,buttheremustbeatwo-waychanneltofeedbackerrorinformationtothesender.Atthesametime,thesendershouldsetupadatabuffertostorethesentdatasothatthewrongdatacanbere-sent.

IntheFECmode,thereceivingendcannotonlyfinderrors,butalsodeterminethelocationofthebinarycodeelementwheretheerroroccurred,soastocorrectit.TheFECmethoduseserrorcorrectioncodes,anddoesnotrequireareversechanneltotransmittheinformationrequestedforretransmission,andthesenderdoesnotneedtostorethedatabufferforretransmission.However,thecodingefficiencyislowandtheerrorcorrectionequipmentisalsomorecomplicated.

Errorcontrolcodescanbedividedintoerrordetectioncodesanderrorcorrectioncodes.

Theerrordetectioncodecanonlydetecterrorsinthetransmission,andthesendercanonlycorrecttheerrorsbyretransmittingthedata;whiletheerrorcorrectioncodecannotonlydetecttheerrorsbutalsoautomaticallycorrecttheerrors,avoidingretransmissions.

Generalerrordetectioncodesare:paritycheckcode,cyclicredundancycode.

Inactualcommunicationnetworks,differenterrorcontroltechnologiesareoftenusedindifferentapplications.Forwarderrorcorrectionismainlyusedinwiredandwirelesstransmissionswithpoorchannelqualityandstricttransmissiondelayrequirements;errordetectionisoftenusedinthemonitoringandmanagementofchannelswithhightransmissionqualityorafterforwarderrorcorrection>Automaticretransmissionrequestsaremostlyusedinfiletransfersthatrequirelowlatencybutveryhighdatareliability,suchascomputercommunications.

Контролен метод

Метод за контрол на грешките

  1. BitErrorRateBitErrorRatePe=NumberofErrorCodesAccepted/TotalAcceptedTherearetwomeasurestakenforthenumberofsymbols:improvingtheelectricalcharacteristicsofthelineandadoptingerrorcontroltechnology

  2. Thecommonerrorcontrolmethodforerrorcontrolistoadderrorcontrolcodestothedata.Beforetheinformationbitstobesent,acertainamountofredundantbitsisaddedaccordingtoacertainruletoformacodewordandthentransmitted.Thereareusuallyfeedbackretransmissiontechnologyandforwarderrorcorrectiontechnology:1)FeedbackretransmissiontechnologyThesenderaddsanerrordetectioncodetotheinformationbit,andthereceiverusestheerrordetectioncodetoperformerrordetectionontheinformationbitafterreceivingthecodeword,ifitiscorrectThenitsendsbackanerror-freesignalandstartstotransmitthenextinformationbit.Ifitisincorrect,itsendsbackanerrorsignal,andthesenderretransmitstheinformationuntilthereceiverconfirmsthatitiscorrect.2)ForwarderrorcorrectiontechnologyThesendingendsendsdatathatcanbecorrected.Afterreceivingthedata,thereceivingendcannotonlyautomaticallyfinderrors,butalsoautomaticallycorrecterrorsintransmission.Theadvantageisthatthereisnoneedforafeedbackchannel,buttheequipmentiscomplicated.

Състав и принцип на действие на системата

Състав на системата

ThecompositionandprincipleoferrorcontrolsystemareshowninFigure1.ThepartwithinthedottedlineinFigure1istheerrorcontrolsystemindigitalcommunication.Whenthereisnoerrorcontrol,thedigital(alsocalledsymbolorsymbol)sequenceoutputbythesourcewillbedirectlysenttothechannel.Duetointerferenceinthechannel,errorswilloccurintheoutputofthechannel.Theprobabilityofdigitalerrorsintransmission(biterrorrate)isamajorindicatoroftransmissionaccuracy.Afterthechannelisgivenindigitalcommunication,ifthebiterrorratecannotmeettherequirements,errorcontrolmustbeadopted.Accordingtodifferentspecificimplementationmethods,errorcontrolcanbedividedintothreetypes:forwarderrorcorrectionmethod,feedbackretransmissionmethodandhybridmethod.Errorcontrol

ForwarderrorcorrectionmethodTheerrorcontrolsystemonlyincludesthechannelencoderanddecoder.Thedigitalsequenceoutputfromthesignalsourceiscodedinthechannelencoder(seechannelcoding),andthensenttothechannel.Sincethechannelencoderusesanerrorcorrectioncode,thedecodercancorrectmostoftheerrorscausedduringtransmissionsothatthesinkcangetamorecorrectsequence.

Aftertheforwarderrorcorrectiondetectsthereceivedsymbolerroratthereceivingend,itdeterminesthespecificlocationoftheerrorthroughacertaincalculationandautomaticallycorrectsit.Alsoknownasautomaticerrorcorrection,itisaneffectivemeanstoimprovechannelutilization.Thechannelcodingtheoryininformationtheoryistostudythelimitoftheforwarderrorcorrectioncapabilityofagivenchannel,andtheerrorcorrectioncodingtheoryistostudythespecificcodinganddecodingmethodoferrorcorrectioncodesforforwarderrorcorrection.Traditionalerrorcorrectioncodingtheorybelievesthatinorderforacodetohaveerrorcorrectioncapability,extrasymbolsmustbeaddedtotheoriginalcodewordtoexpandthedifferencebetweencodewords(calledcodedistance).Itisgenerallybelievedthattheextrasymbolsaddedduetoerrorcorrectionduringtransmissionwillincreasetheoccupiedbandwidthandreducethetransmissionrateperunitbandwidth.Thetheoryofcombinedcodingandmodulationistoconsidermodulationanderrorcorrectioncoding.Byexpandingthesetofmodulatedsignals,theerrorcorrectioncapabilityofthecodingandmodulationsystemcanbeimprovedwithoutincreasingtherequiredchannelbandwidth.Forwarderrorcorrectionhasbeenwidelyusedinsatellitecommunications,mobilecommunications,andfrequencybanddatatransmission.

Thefeedbackretransmissionmethodonlyusestheerrordetectioncodetofindtheerrorscausedbythetransmission.Atthesametime,aftertheerrorsarefound,thesenderisnotifiedtoretransmitthecorrespondingsetofnumbersthroughthereversechanneltoimproveAccuracyoftransmission.Accordingtodifferentretransmissioncontrolmethods,thefeedbackretransmissionmethodcanalsobedividedintoseveralimplementationmethods.Thesimplestoneiscalledwaitingforretransmission.Inthisway,thesenderstopsandwaitsfortheanswerfromthereceivereverytimeitsendsasetofnumbers.Atthistime,thechanneldecodersendsacorrectanswertothesenderthroughthereceiverretransmissioncontrollerandthereversechannelifnoerrorisfound.Afterreceivingit,thesenderusesthesenderretransmissioncontrollertocontrolthesourcetotransmitthenextsetofnumbers,otherwisethesourcewillretransmittheoriginalsetofnumbers.

FeedbackretransmissionAfterthereceiverdetectsanerrorinthetransmission,itwillautomaticallynotifythesendertoretransmittheerrormessage.Differentfromforwarderrorcorrection,theautomaticretransmissionrequestcandynamicallyadjustthetransmissionrateaccordingtothechangeofchanneltransmissionqualityundertherequirementofafixederrorrate.Itisanadaptiveerrorcontrolmethod,buttheremustbeAfeedbackchannel.Automaticrequestretransmissionisthemostwidelyusedinend-to-enderrorcontrolthatrequiresstricterrorcodes.

Themaindifferencebetweentheabovetwomethodsis:①Forwarderrorcorrectiondoesnotrequireareversechannel,whilefeedbackretransmissionmusthaveareversechannel.②Forwarderrorcorrectionuseserrorcorrectioncodes,andfeedbackretransmissionuseserrordetectioncodes.Generallyspeaking,theimplementationoferror-correctingcodesismorecomplicated,withfewererrorsthatcanbecorrected,whiletheimplementationoferror-detectingcodesiseasierandmoreerrorscanbefound.③Themessagedelaycausedbyforwarderrorcorrectionisfixed,andtherateofmessagetransmissionisalsofixed.Themessagedelayandmessagetransmissionrateinfeedbackretransmissionwillchangewiththechangeofretransmissionfrequency.④Forwarderrorcorrectiondoesnotrequirecontrolofthesource,whilefeedbackretransmissionrequirescontrolofthesource.⑤Theaccuracyofthetransmittedmessageafterforwarderrorcorrectionwillstillchangegreatlywiththechangeofchannelinterference,whiletheaccuracyofthetransmittedmessageafterfeedbackandretransmissionisrelativelystable,andgenerallydoesnotchangewiththechangeofinterference.Therefore,theapplicationoccasionsofthetwoareverydifferent.

Inthehybridmethod,whenthechannelinterferenceislarge,thetransmissionrateofthemessagewilldroptoomuchduetothecontinuousretransmissionduetothefeedbackretransmissionalone,andtheforwarderrorcorrectionalonecannotguaranteesufficientaccuracyAtthistime,itismoreadvantageoustouseboth,thisisthehybridmethod.Thechannelcodingusedinthismethodisacodethatcannotonlycorrectsomeerrorsbutalsofindmosterrors.Thechanneldecoderfirstcorrectsthoseerrorsthatcanbecorrected,andonlyrequiresretransmissionsforthosethatcannotbecorrectedbutcanbefound.Thiswillgreatlyreducethenumberofretransmissions.Atthesametime,duetothestrongerrordetectioncapabilityofthecode,theaccuracyofthefinaldigitalmessageisrelativelyhigh.

Роля

Errorcontrolhasbeensuccessfullyappliedtosatellitecommunicationsanddatacommunications.Insatellitecommunication,convolutionalcodesorconcatenatedcodesaregenerallyusedforforwarderrorcorrection,whileindatacommunication,blockcodesaregenerallyusedforfeedbackandretransmission.Inaddition,errorcontroltechnologyisalsowidelyusedincomputers.Thereareroughlytwospecificimplementationmethods:①Usingerrorcorrectioncodestoautomaticallycorrecterrorsgeneratedbyhardware;②UsingerrordetectioncodesthroughrepeatedexecutionofinstructionsorprogramsaftererrorsarefoundPartialreturntoeliminateerrors.

Related Articles
TOP